In a full duplex transceiver analog signals are simultaneously transmitted and received on a cable. For example, Gigabit Ethernet transceivers simultaneously transmit and receive analog signals over twisted pair category 5 copper cable. In full duplex operation the receive signal contains both the signal from the remote end transmitter and the signal from the local transmitter. The signal from the local transmitter is referred to as echo. In order to correctly recover the data from the remote end transmitter in the receive signal it is necessary to cancel the echo in the receive signal. This task may be accomplished with an echo canceller circuit.
An echo canceller circuit works on the principle of subtracting an estimate of the transmit signal from the full duplex signal. The full duplex signal that reaches the receiver comprises the receive signal plus echo signals from the transmit signal. An echo canceller circuit provides a copy of the transmit signal to the receiver portion of the transceiver so that the receiver can subtract the transmit signal from the full duplex signal.
Analog echo cancellation is very important. Analog echo cancellation provides a large amount of echo cancellation. This reduces the complexity and the size of any digital echo canceller that may be used to obtain additional echo cancellation after the analog signals are converted to digital signals. More importantly, analog echo cancellation produces a high signal to echo ratio at the input of an analog to digital converter (ADC). This means that the receive signal can be boosted before entering the ADC without saturating the ADC. This additional boost leads to less quantization noise and a higher signal to noise ratio (SNR). In this manner, good analog echo cancellation can significantly improve the bit error rate and cable length performance of the transceiver.
The transmit signal that is actually placed on the cable by the local transmitter of the transceiver (and therefore the echo) depends upon the impedance that is presented to the integrated circuit (IC) that contains the transmitter of the transceiver. The impedance that is presented to the IC is made up of a number of elements including the impedance of the cable, the z11 impedance of the transformer that couples the cable to the transceiver front end, the impedance of the IC on-board traces, and the impedance of the IC input/output (I/O) structures such electrostatic discharge protectors, I/O cells, and the like. The effective impedance that is presented to the IC will vary depending upon the variations within these components and the manufacturing tolerances within these components.
There is therefore a need in the art for an improved echo canceller circuit that is capable of taking into account the actual external impedance that is encountered by a transmit signal in a full duplex transceiver.